In radar and communications systems, demands for efficient wideband radio frequency transmitters gradually increase. Critical factors affecting efficiency of a radio frequency transmitter include battery costs, an energy expense of an electrical communications system, a weight of a power supply, an operating time, a size of a cooling system, and the like. Improvement of efficiency of a power amplifier, which is an important part of a radio frequency transmitter and a radio frequency receiver, is of crucial importance.
Efficiency of a power amplifier refers to a ratio of output power of the power amplifier to power consumption of the power amplifier. Generally, a voltage and a current of the amplification device need to be shaped, to achieve high power and high efficiency. In a communications system, generally an efficiency enhancement technology based on a Doherty circuit structure is used. This technology can greatly increase efficiency at a back-off point. FIG. 1A is a schematic structural diagram of a typical Doherty power amplification device 100. The Doherty power amplification device 100 mainly includes a power divider 102, which receive the radio frequency input, a primary amplifier 104, a secondary amplifier 106, and various microstrips 108, 112, 114, and 116 for impedance matching. The primary amplifier 104 is also referred to as a main amplifier, the secondary amplifier 106 is also referred to as a peak amplifier, and both the primary amplifier 104 and the secondary amplifier 106 operate in a fundamental frequency band. When power of an input signal is lower than an enabling threshold, the secondary amplifier 106 is turned off, the primary amplifier 104 operates in a high-impedance state, and back-off efficiency at a saturation point is relatively high. When power of an input signal is greater than an enabling threshold, the secondary amplifier 106 is turned on, and a fundamental signal that is output by the secondary amplifier 106 and a fundamental signal that is output by the primary amplifier 104 are combined using an output network to achieve an effect of load pulling.
To further improve efficiency of the Doherty power amplification device 100, a harmonic injection technology may be introduced to the Doherty power amplification device 100. However, because the Doherty power amplification device 100 includes at least two amplifiers, if one or more harmonic amplifiers are further introduced, a circuit structure of the Doherty power amplification device 100 becomes quite complex, and costs and an area also become unacceptable. Therefore, how to design a power amplification device having a simple structure and relatively high efficiency is a problem that needs to be urgently resolved.